robson



Sept. 30,1958 T. s. ROBSON TUNABLE CIRCUITS FOR HIGH FREQUENCIES 2Sheets-Sheet 1 Filed Jan. 23, 1956 F1603 ll'Z/l) 1 5120276022,

Sept. 30, 1958 'r. $.'ROBSON- TUNABLE cmcuns FOR HIGH FREQUENCIES 2Sheets-Sheet 2 Filed Jan. 23, 1956 men/2291 276.1530 bqjolz/ UnitedStates Patent TUNABLE CIRCUITS FDR HIGH FREQUENCIES Application January23, 1956, Serial No. 560,844

Claims priority, application Great Britain January 27, 1955 6 Claims.(Cl. 179-171) This invention relates to tunable circuits for highfrequencies and especially, though not exclusively to such circuits asused with disc seal valves.

When it is desired to amplify electrical signals of ultrahigh frequencyso-called disc seal valves are often employed, the valves having inputand output circuit in the form of co-axial line circuits. In order thata valve in such an arrangement can be replaced quickly it is usual toarrange that the anode circuit is folded back over the input circuit asdescribed in U. S. patent specification No. 2,235,414. A disadvantage ofthis arrangement is that the input circuit is inaccessible and it isdifficult to couple to the input circuit without increasing its lengthand making it operate with an electrical length containing more quarterwavelengths than would otherwise be necessary. In the event of the inputcircuit being operated as an open circuited line it is diificult tobring out the cathode and filament leads.

To maintain the advantage of the folded anode line circuit whilstrendering the input circuit more accessible it has been proposed toconstruct the anode circuit of two co-axial tubes both of considerablygreater diameter than the outer tube of the input circuit. By thisexpedient, space is left between the two circuits for any couplingprobes which are necessary. When the anode circuit is constructed inthis way it will be appreciated that the anode circuit is in the form ofa folded cavity resonator having, when considered in cross-section, twoparts which though adapted to resonate as a single cavity are at rightangles to each other, one being an annular part formed by the twoco-axial tubes and the other being an inner part formed by discs whichconnect the inner ends of these tubes respectively to the controlelectrode and to the anode. It is usual to tune the anode circuit of adisc seal valve by means of a short circuiting ring which is disposedbetween the two conductors and can be moved along the axis of theconductors and so vary the electrical length of the resonator lineformed thereby. However when both limbs of a folded anode circuit are ofappreciable length, such tuning means introduce the disadvantage thatthe frequency range is limited by the inability of the short circuitingring to come closer to the valve than the end of the two co-axial tubes.Therefore the highest frequency which it should be possible to reachwith a given construction is not attainable,

The object of the present invention is to reduce this last-mentioneddisadvantage and according to the present invention there is provided atunable electrical circuit comprising a cavity resonator which is formedin at least two parts adapted to resonate as a single cavity anddisposed at an angle to each other, and means for virtually varying adimension of each of said parts to tune said resonator.

In one form of the invention, the resonator compnses an annular part andan inner part extending towards the axis of the annular part from oneend thereof, and in this form of the invention the means for varying ad1- ICC 2 mension of each part of the resonator may comprise tuningmeans movable in an axial direction in said annular part and othertuning means movable in a radial direction in said inner part.

In order that the invention may be clearly understood and readilycarried into elfect, the invention will be described with reference tothe accompanying drawings, in which:

Figure 1 is a sectional elevation of an ultra-high frequency thermionicvalve circuit employing a tuned circuit according to the presentinvention. 7

Figure 2 is a plan view of Figure 1 with the top cover of the anodecircuit removed, and

Figure 3 illustrates a modification of Figure 1.

Referring to Figure 1, reference 1 represents a disc seal thermionicvalve having an anode 2, control electrode 3 and a cathode 4. The inputcircuit of the valve is formed of two co-axial conductors 5 and 6 whichare connected, .for high frequency currents, to the control electrodeand cathode respectively of the valve 1. The signals to be amplified inthe valve are injected into the input circuit by a coupling probe 7. Theanode circuit of the valve comprises two concentric tubes 8 and 9 bothof considerably greater diameter than the outer tube 5 of the inputcircuit so that a space is left between the two circuits for thecoupling probe 7. The upper ends (in the drawing) of the tubes 8 and 9'are connected for high frequency currents to the control electrode andanode respectively of the valve 1. The connections are efiected byco-axial discs 10 and 11, which have flanges 14 and 15 at their innerends. These flanges are conductively connected to the anode 2 andcontrol electrode 3 so that polarising potentials can be applied to theelectrodes, but the flanges are insulated from the discs 10 and 11 fordirect current by insulating rings as indi cated. The anode circuit istherefore in the form of a cavity resonator comprising an annular partand an inner part extending towards the axis of the annular part fromone end thereof, the inner part of the resonator being apertured so thata valve can be fitted to the resonator with electrodes of the valveelectrically coupled to the Walls of the inner part of the resonator.The anode circuit is tunable by a sort circuiting ring 12 which ismovable in known manner in an axial direction along the annular part ofthe resonator formed by the tubes 8 and 9, so as to vary the electricallength thereof. However the anode circuit has further tuning means inthe form of short circuiting bars 13 which are disposed in the innerpart of the resonator, namely between the discs 10 and 11 at angularlyspaced positions about the axis of the valve 1, these bars being movablein a direction perpendicular to the axis of the tubes 8 and 9 towards oraway from the valve. In the present example the bars 13 are movable inthe radial direction. The number of short circuiting bars 13 is notcritical, three or four being for example suitable. In the present casefour short circuiting bars are employed, as can be seen in Figure 2. Thebars are curved in the present example but may alternatively be straightand when in their in ner most positions they may either form a closedring, a closed square or some other configuration, close to and aroundthe valve.

When tuning the anode circuit, for the range of frequencies covered bythe short circuiting ring 12, the bars 13 are retained in their extremeouter positions as shown in Figure 1. When the short circuiting ring 12is in the position nearest to the valve, namely the position indicatedby dotted lines in Figure 1, the anode circuit can be tuned to a stillhigher frequency by moving the bars 13 towards the valve. Th highestfrequency to which the circuit can be tuned is therefore, by virtue ofthe 3 invention, limited only by the size of the valve and its physicalconstants.

In the modification shown in Figure 3, the co-axial tubes 8 and 9 of theanode circuit are disposed above the valve so that the anode circuit isnow in effect folded upwardly. The diameter of the inner tube 8 in thiscase has to be large enough for the valve to be replaced through it.This arrangement leaves the input circuit even more accessible than inthe arrangement shown in Figure l.

The tuning mechanism for the circuit may include mechanical andelectrical interlocking means for indi' cating which of the two tuningmeans should be employed, and to prevent the wrong one being used.

In one practical form of the invention, the valve 2 was an American typevalve 4X150G. The short circuiting ring 12 enabled the resonantfrequency and the anode circuit to be varied from 250 to 700 mc./s.,with the resonator operating in the fundamental or lambda/ 4 mode. Theshort circuiting bars 13 then enabled the frequency to be varied from700 mc./s. to 960 mc./s., the combined lengths of the two parts of theresonator being one foot long in-this example. Therefore by virtue ofthe invention it was possible to tune over a frequency range from 250 to960 mc./s. with a comparatively compact circuit, operating as aforesaidin the fundamental mode.

What I claim is:

l. A tunable electrical circuit comprising a cavity resonator having anannular part and an inner part extending towards the axis of the annularpart from one end thereof, said parts being adapted to resonate as asingle cavity, tuning means movable in an axial direction in saidannular part and other tuning means movable in said inner part in adirection towards and away from the axis of the annular part.

2. A circuit according to claim 1 wherein said inner part is formed withan aperture and a thermionic valve is fitted in said aperture withelectrodes of the valve electrically coupled to the walls of said innerpart.

3. A circuit according to claim 2 comprising a further cavity resonatorelectrically coupled to electrodes of said valve and disposed at leastpartly within the annular part of said first resonator, being spacedfrom the inner wall of said annular part, one of said resonators formingan input circuit for said valve and the other resonator forming anoutput circuit from said valve.

4. A circuit according to claim 2 comprising a further cavity resonatorelectrically coupled to electrodes of said valve and projecting to theopposite side of the inner part of the first resonator from the annularpart thereof, one of said resonators forming an input circuit for saidvalve and the other resonator forming an output circuit from said valve.

5. A circuit according to claim 2 comprising a further cavity resonatorelectrically coupled to electrodes of said valves and disposedco-axially with the annular part of said first cavity resonator, saidfurther cavity resonator being spaced from said annular part, one ofsaid resonators forming an input circuit for said valve and the otherresonator forming an output circuit from said valve.

6. A circuit according to claim 1 each of said tuning means comprisingshort circuiting means movable within the respective parts.

References Cited in the tile of this patent UNITED STATES PATENTS2,525,452 Gurewitsch Oct. 10, 1950 2,554,501 Preist May 29, 19512,641,657 Preist June 9, 1953

